Technical Field
The present invention generally relates to sensors and methods for sensing a gaseous analyte.
Background Information
Gas detection instruments or sensors have a wide range of applications, including industrial health and safety, environmental monitoring, and process control. Sensors are used in a variety of fields, including chemical and petroleum refining, rocket fuel production, fuel cell manufacturing, semiconductor processing, and biomedical applications. For example, nanoscale sensors can be used to detect certain gaseous analytes in a sample, such as human exhaled breath. The presence and concentration of particular analytes may be used to diagnose various diseases.
Nanoscale sensors comprise nanomaterials such as carbon nanotubes (CNTs). Nanomaterials can exhibit sensitivity to gases. For example, in view of their unique electrical, thermal, and mechanical properties, CNTs can be used to make gas sensors. Other nanomaterials have also shown promise for use as gas sensors.
The sensing mechanism of nanomaterial-based gas sensors depends either upon charge transfer between the nanostructure building blocks or, due to adsorption of charged or polar molecules of the gases on the surfaces of the nanostructure building blocks. An electron donation or withdrawal due to adsorption of the gas analytes changes the conductivity of the nanomaterials. Nanomaterial-based sensors, therefore, using low-power microelectronics technology, convert chemical information into an electrical signal, leading to the formation of miniaturized sensor devices.
CNTs and other nanostructures, such as nanowires and nanodots, have been demonstrated as appealing sensing materials for developing advanced chemical gas sensors. Based on the mechanism of charge transfer, gas adsorption (for example, nitrogen dioxide (NO2), ammonia (NH3), and oxygen (O2)) can cause significant electrical transport property changes in the CNTs and nanowires and nanodots, which can be beneficial for sensing applications.
However, gas sensors based on bare nanomaterials, such as pristine CNTs, have limitations, including low sensitivity (due, for example, to low absorption capacity), and a lack of selectivity to analytes for which they have low adsorption energy or low affinity. This less-than-ideal sensitivity and selectivity has limited the use of nanomaterial-based gas sensors in practical applications. Efforts have been made to improve gas sensitivity and selectivity of CNTs by functionalizing the CNTs with analyte-specific materials. However, functionalization of the sensors can require long fabrication time and complicated fabrication steps, which can make the process complex and costly.
Conducting polymers represent one type of sensitive material that has been investigated for CNT functionalization. For example, polyaniline (PANT) has been used as a sensing material for a variety of toxic gases such as carbon monoxide, nitrogen dioxide, and ammonia. PANI exhibits p-type semiconductor characteristics, so electron-supplying gases such as ammonia reduce the charge-carrier concentration and decrease the conductivity of the polymer. However, CNT functionalization can involve complex fabrication processes, and resulting functionalized CNTs can lack the desired properties for sensing applications.
A need exists for improved CNT gas sensors and methods that do not require a complex fabrication process or high operating temperatures, and that have the desired properties for sensing applications, including, for example, low resistivity and high resolution, specifically at low analyte concentrations.